An extrusion process referred to by the name of paste or lubricated extrusion is known for the manufacture of hollow bodies such as tubes made of fluoro resin, especially of polytetrafluoroethylene (PTFE).
This process comprises stages consisting in mixing a fine powder of unsintered fluoro resin containing approximately 20% of lubricant, in compressing this mixture to obtain a compact tubular preform, and in introducing this preform into an extrusion press in which a ram expels the preform through a die to produce a tubular section.
This process makes it possible to produce fibres or fibrils of fluoro resin, these fibres being naturally oriented in the direction of the flow of material, in other words in the lengthwise direction of the body, as a result of the small section of the die exit orifice.
This lengthwise orientation of the fibres is generally detrimental from the viewpoint of mechanical strength, because it promotes the formation of lengthwise cracks when the body begins to burst, especially in the case of tubes subjected to an internal pressure.
An anisotropy ratio A substantially equal to 2 is generally obtained for a tube manufactured in this way. ##EQU1##
For a hollow body under pressure to exhibit substantially the same risks of cracking in the lengthwise and transverse directions it is necessary for the transverse tensile strength to be twice the lengthwise tensile strength, that is to say for the anisotropy ratio A of the hollow body to be substantially equal to 0.5.
Numerous techniques are known which make it possible to reduce the anisotropy ratio of a product made of PTFE, obtained by lubricated extrusion.
A first technique consists in varying the reduction ratio R of the die. ##EQU2##
The decrease in the reduction ratio of the die makes it possible to reduce the anisotropy ratio.
An example of this technique is provided by document U.S. Pat. No. 3,315,020, which permits the manufacture of biaxially oriented thin sheets which are substantially isotropic, that is to say exhibiting an anisotropy ratio close to 1.
Although these modifications make it possible to reduce the anisotropy ratio to the vicinity of 1, they frequently take place to the detriment of the absolute value of the tensile strength of the product obtained.
A second technique consists in subjecting the product to reheating which makes it possible to reduce the previous orientation of the fibres and therefore to obtain, here again, an anisotropy ratio A close to 1. However, this technique is particularly costly.
A third technique consists in employing highly modified powders which are therefore much more expensive than standard powders.
However, whatever the technique employed, it is currently impossible to obtain by paste extrusion products made of PTFE, and in particular hollow bodies, exhibiting an anisotropy ratio A substantially lower than 1.